Compared with corresponding emulsion rubbers, anionically polymerized solution rubbers containing double bonds, such as solution polybutadiene and solution styrene/butadiene rubbers, have advantages in the manufacture of tire treads with a low rolling resistance. The advantages lie inter alia in the ability to control the vinyl content and the associated glass transition temperature and the molecular branching. This gives rise in practical use to particular advantages in the relationship between the wet grip and the rolling resistance of the tire. Thus, U.S. Pat. No. 5,227,425 describes the manufacture of tire treads from a solution SBR and silica. Numerous methods of end group modification have been developed to improve the properties further, e.g., with dimethylaminopropylacrylamide as described in EP-A 334,042 or with silyl ethers as described in EP-A 447,066. Because of the high molecular weight of the rubbers, however, the proportion by weight of the end group is small and cannot, therefore, greatly influence the interaction between filler and rubber molecule. One object of the present invention is to prepare solution SBRs with a markedly higher content of effective groups for interaction with the filler.
Solution polybutadiene rubbers containing hydroxyl groups are also described in DE-OS 2,653,144. However, because their strength is too low, these rubbers are not suitable as the main component in tire treads.
EP-A 464,478 describes a process for the hydroxylation of rubbers, but this involves the introduction of secondary hydroxyl groups, which are far less effective than the primary hydroxyl groups of the present invention.
Emulsion and solution rubbers containing hydroxyl groups are also described in EP 806,452 A1, the hydroxyl contents described in this case for solution rubbers being in an appreciably lower range (0.009 to 0.061%) as a consequence of the process. The present invention shows that these contents have no significant effect on the wet grip.